A polymer electrolyte type fuel cell refers to a fuel cell using a proton conductive polymer as an electrolyte that takes out energy from a fuel such as hydrogen, methanol or the like by converting chemical energy of the fuel into electrical energy, by electrochemically oxidizing the fuel by means of oxygen or air. The polymer electrolyte type fuel cells include a type that uses pure hydrogen supplied from a steel bottle, pipe or the like as a fuel, and a type that generates hydrogen from gasoline or methanol using a reformer and uses the hydrogen as a fuel. Furthermore, there has also been developed a direct methanol type fuel cell (DMFC) that directly generates power using an aqueous methanol solution as a fuel. The DMFC, which does not require a reformer for generating hydrogen, can have a simple and compact system and has been gathering attention particularly as a power supply for portable equipment.
The polymer electrolyte type fuel cell is composed of a polymer electrolyte membrane, and positive and negative electrodes that are arranged so as to be in contact with both sides of the polymer electrolyte membrane. Hydrogen or methanol as a fuel is electrochemically oxidized at the negative electrode to generate protons and electrons. The proton moves through the polymer electrolyte membrane to the positive electrode where oxygen is supplied. On the other hand, the electron generated at the negative electrode flows into the positive electrode via a load connected to the fuel cell, and water is generated by reaction of the proton with the electron at the positive electrode. For this reason, high proton conductivity is required for a polymer material used for the electrolyte membrane, a binder for binding the membrane and the electrodes, a binder for fixing a catalyst that accelerates oxidation of hydrogen or methanol and reduction of oxygen, or the like. Further, the electrolyte membrane requires properties for shielding hydrogen or methanol as a fuel. However, conversely, a binder for fixing a catalyst for an electrode requires properties to allow methanol to permeate, since the fuel needs to be supplied to the catalyst. Meanwhile, when adhesion at an interface between the electrolyte membrane and an electrode or an interface between the catalyst and the binder is insufficient, conduction of the proton is inhibited. Therefore, the polymer materials used for these require high adhesion.
As the polymer material having high proton conductivity, a protonic acid group-containing fluorinated polymer compound such as a product named Nafion (registered trademark, produced by DuPont Kabushiki Kaisha), a polymer membrane, produced by Dow Chemical Co., or the like is known. However, the protonic acid group-containing fluorinated polymer compound has problems such as being very expensive, generating fluoric acid gas when it is burned at the time of disposal, being unsuitable for a polymer electrolyte membrane for DMFC because of low methanol shielding properties of the membrane, and having proton conductivity that rapidly drops under high temperature and low humidity.
On the other hand, non-fluorinated polymer electrolyte materials using a protonic acid group-containing hydrocarbon type polymer compound have also been under development. The protonic acid group-containing hydrocarbon type polymer compound is known for low production cost, no generation of halogen type gas upon incineration, and small decrease in proton conductivity under high temperature and low humidity. However, for example, it is known that sulfonated polystyrene has cell properties that deteriorate over time since a tertiary carbon in its main chain is susceptible to attack by a radical and hydrogen is easily emitted at an α position in a cell.
For this reason, a large number of protonic acid group-containing polymer compounds which do not have an aliphatic chain in a main chain, that is, aromatic hydrocarbon type polymer compounds, have been developed (for example, Non-patent Document 1). Among these compounds, it has been reported that a membrane composed of sulfonated aromatic polyether is excellent in heat resistance and chemical durability so that it can be used as a polymer electrolyte membrane for a long time. Further, a crosslinked membrane of sulfonated aromatic polyether in which inter-molecular chains are crosslinked has excellent water resistance and methanol solubility resistance, satisfying both methanol shielding property and proton conductivity at the same time. Thus, it is suitable for use in a polymer electrolyte membrane for DMFC (for example, Patent Document 1).
However, a fuel cell using a protonic acid group-containing aromatic hydrocarbon type polymer compound has a problem of deterioration of cell properties due to fluctuation in humidity or temperature. This is considered to be because of detachment at an interface of a membrane and an electrode or an interface of a catalyst and a binder, resulting from repeated expansion and contraction of the proton conductive material caused by fluctuation in the humidity or temperature. Such a problem is conspicuous, in particular, when a protonic acid group-containing fluorinated polymer compound is used as a binder. Since the glass transition temperature of the protonic acid group-containing fluorinated polymer compound is low, about 140° C. in the case of Nafion, a membrane and an electrode can be tightly heat-fused by heat pressing when the polymer electrolyte membrane is a protonic acid group-containing fluorinated polymer compound. However, when the polymer electrolyte membrane is a protonic acid group-containing aromatic hydrocarbon type polymer compound, detachment at an interface easily occurs due to low affinity with the protonic acid group-containing fluorinated polymer compound in the binder.
Methods for preventing detachment include use of a membrane having strong adhesion or a binder having high adhesion. As a membrane having strong adhesion, a membrane with a reformed surface, e.g. a membrane with a roughened surface (for example, Patent Document 2), a membrane with a surface hydrophilized by performing a discharge treatment (for example, Patent Document 3) and the like have been reported. However, sufficient effects of improving adhesion have not been achieved.
On the other hand, several polymer electrolyte membranes or binders using a protonic acid group-containing aromatic hydrocarbon type polymer compound are known (for example, Patent Documents 4 and 5). However, polymer compounds described in these documents have a glass transition temperature of 200° C. or more. Therefore, when a polymer compound having such a high glass transition temperature is used as a binder, there is a problem in that the binder cannot be attached to an electrode if the temperature is not high. On the other hand, the protonic acid group has low thermal stability and is eliminated at relatively low temperature, and as a result, there is a problem such that it cannot be strongly melt-adhered to an electrode. For this reason, a binder using a protonic acid group-containing aromatic hydrocarbon type polymer compound having good adhesion has been demanded.
An object of the present invention is to provide a binder for a fuel cell having excellent adhesion, high methanol permeability and high proton conductivity. Furthermore, the present invention provides a composition for forming a fuel cell electrode using the binder, an electrode for a fuel cell, and a fuel cell.
Patent Document 1: WO 03/0033566
Patent Document 2: Japanese Patent Application Laid-open No. 2003-317735
Patent Document 3: Japanese Patent Application Laid-open No. 2002-237315
Patent Document 4: Japanese Patent Application Laid-open No. 2004-359925
Patent Document 5: Japanese Patent Application Laid-open No. 2004-47244
Non-patent Document 1: Macromol. Chem. Phys., Vol. 199, pp. 1421-1426 (1998)